This invention relates to a color television viewing device and, more particularly, to such a device which receives standard color television signals, reproduces separate color images from those signals, and optically recombines the signals into a composite colored image.
In the past, several different systems have been attempted which optically recombine separately displayed primary color pictures of a single scene. In such systems, the three primary colored pictures of the scene are presented on the face of a cathode ray tube. (See for example U.S. Pat. Nos. 2,552,464, 2,603,706 and 2,600,590). In all such systems, special television transmission systems or cameras must be used which are compatible with the viewing devices. Another common problem with such systems is that in optically recombining the three separate colored pictures, they are projected onto a screen. Screen projection both limits the size of the image and its intensity.
Some prior art optical recombining systems use a single objective lens to reduce both the cost of the system and the distance which is necessary between the objective lens and the display screen, but there is the problem of equalizing the distance of the optical paths between the objective lens and the display screen. If the optical path lengths are different, there will not be proper registration upon recombination of the images at the display screen. While attempts have been made to overcome this problem, they have not proved feasible where the cathode ray tube is operating from standard color television signals generated by commercially available cameras and transmitted through conventional television transmission networks. See for example U.S. Pat. No. 2,600,590 where the colored images are placed at equal distances from the objective lens in a radial arrangement with respect to the optical axis.
With conventional color television, which uses a standard color television cathode ray tube, there has been the problem that such devices must be of a relatively large size. One reason that this has been the case is that all color tubes now in commercial use are of the shadow mask type. Although there are different shadow mask techniques, it is extremely difficult with any of them to miniaturize the color tube for hand-held viewing use. One of the desirable advantages of a hand-held viewer is that the viewing screen can be viewed through a large diameter lens whose focal length is nearly equal to the distance between the lens and the tube face. The tube face will then appear to be greatly magnified in size and to be some distance away from the observer. The angular width (in radians) of the virtual image stays almost constant and equal to the linear width of the picture, i.e., the tube face, divided by the focal length of the lens as the image moves from a few feet away, to infinity. Small changes in the tube face distance make the virtual image of the television picture move from infinity into a few feet from the observer's eyes.
If the observer moves his or her head to the left or right, or up or down, the observer sees just the parallax that would be seen if the virtual image was a real object, and there was no lens in front of the observer's eyes. Secondly, the observer's convergence eye muscles that give stereo range-finding ability also tell the observer that the image is at its calculated position. Furthermore, if the observer is young, the observer's accommodation senses (cilliary muscles that change the shape of the eye lens) confirm that the virtual image is as distant as it is supposed to be. So, by all three tests (which the observer makes without thinking about them) the picture is big and faraway.
This is to be contrasted with moving a real image of a small television tube face over such a wide range of distances, focusing it always on a wide screen. In doing so, one would find that the apparent brightness of the image drops rapidly as the screen is moved away. Yet, when the virtual image of a small television screen is moved from being very close to the observer, all the way to infinity, its apparent brightness stays constant.
Another important consideration is the cathode ray beam "spot size" as measured as a function of the picture height. As the size of the picture tube gets smaller, the diameter of the spot must go down proportionally, if we demand that the vertical resolution in the picture does not degrade. However, if we want the same screen brightness, we need the same number of electrons hitting per unit area on the screen in a unit of time. Thus, when the spot size shrinks in area, the required beam current decreases as the picture height, squared. Conversely, if the beam current remains the same, the picture becomes brighter as the picture size shrinks in area. This analysis leaves out a number of parameters that influence screen brightness, but in general, it is easier to produce a high resolution, bright picture in a small size than it is in large sizes.